I've used it a few times when i've had either a slow leak or a nail/screw in a tyre that's past its prime but still has a lot of life left in it, and in fairness it's served me very well a couple of times by not deflating when i've had the misfortune of picking up another nail/screw and been able to ride to a tyre shop with a still fully inflated tyre.
Pro's:
Will get you out of shit when you least expect it
Cheap enough to buy then get the most out of a partly worn tyre with life left in it
Seals slow leaks, within the tread pattern
Easy to put in tyre (better with a proper compressed air generator though, rather than foot pump, although both will work okay)
Con's:
Tyre fitters don't particularly like it, but so what? They don't mind selling you the stuff, so it's not really your problem and it's there to do a job. Tyre fitters are not your best mates, so don't worry about it and just let them get on with their job.
Tyre fitters WILL NOT repair a tyre with this stuff in. And after the last time when i took my tyre to be changed and saw green shit all up his walls when he took the valve out then i can totally understand why. It's called Slime for a reason, so to clean an area to glue a patch on would be difficult, hence the reason if i had a puncture in a newish tyre i'd get it plugged and not use this stuff.
It's really designed as a temporary fix, but as i said it is good to plug a slow leak then just ignore it.
Problem is i found it goes off after a couple of years and turns into more liquid than slime. So last time, not only did it not plug the hole that a nail caused, forcing me to plug it temporarily myself, but because it was much more fluid over time it really knocked the balance out of the rear tyre to the point of almost making the rear end wobble. I thought my rear shock was on its way out going around corners.
So in summary, if you want a quick fix whilst saving up for a few months for a new tyre, or if you're going somewhere where a puncture could become a very big problem then i would highly recommend it.
But a few months is all it's worth using for.